In a typical cellular telephone system, an area is divided into a plurality of cells with each cell having a centrally located cell site. A mobile unit moving in such a cellular network communicates by radio with a nearest cell site. The cell sites are each connected by cable or point-to-point microwave to a telephone network interface. The network interface typically provides communication among cell sites and between the cell sites and the so-called wire line telephone network. The functions of a typical network interface are described in The Bell System Technical Journal, January 1979, Volume 58, No. 1. One of the functions to be performed by the telephone network interface is the so-called “handoff” function. As a mobile unit moves through a cellular network, it will move away from one cell site and toward another cell site. Each cell site monitors signal quality of the signal received from the mobile unit and passes information to the telephone network interface and determines when a call in progress is to be transferred from one cell site to another. This procedure is known as “handoff”. The handoff process involves several operations including selecting a cell site trunk between the MTSO and the new cell site, sending a message to the mobile unit transmitter/receiver to tune from its present voice channel to a voice channel in the new cell site corresponding to the newly selected trunk, setting up a talking path in the MTSO from the cell site trunk to the trunk of the telephone network presently in use in the call, and idling the talking path in the switching network in the MTSO between the old cell trunk and the telephone network trunk assigned to the call.
A problem with existing mobile telephone systems is the considerable time required in handoffs. This becomes a particular problem in urban areas which are highly congested. A basic principle of cellular telephone systems is the concept of frequency reuse. It can be shown that traffic capacity of a cellular system is increased by a factor N.sup.2 as the size of the cell, i.e., its diameter, is decreased by a factor of N. This is due to the fact that, at least in principle, all frequencies in the mobile telephone spectrum are available for use in each independent cell. Thus, as the number of cells is increased, the total number of calls that can concurrently exist in an area is increased. A drawback, however, to decreasing the size of the cells is that a mobile unit tends to cross cell boundaries more often, requiring a larger number of handoffs which will tend to overload the mobile telephone switching office (MTSO) to the point where existing calls may be interrupted or dropped.
Personal communication service (PCS) functions in substantially the same manner as the mobile cellular system. In PCS, the user may be in a building or walking in a street or riding a vehicle and using a handset which communicates with a base station in the same manner that the mobile unit communicates with the base station or cell site in the cellular network. It is envisioned that PCS, by implementing very small cells, could provide service to a very large number of users, for example in a densely populated urban area. The difficulty with PCS is the same as in the cellular system in that handoffs become the bottleneck.
Modern cellular systems use what is known as code division multiple access (CDMA) spread-spectrum communications. In direct-sequence coding CDMA (DS-CDMA), the energy of the user signal is distributed uniformly over the system bandwidth through the spreading process providing separation between users of the same frequency in adjacent cells. A requirement of DS-CDMA is that no interfering signal received may be significantly stronger than the desired signal since it would jam the weaker signal. This type of coding is used in what is sometimes referred to as hierarchal cell structures. The most commonly referenced hierarchal structure is a macro/umbrella cell overlaying a number of micro cells. A fast-moving mobile unit, for example, may be served by the macro cell to avoid an extreme number of handovers. Slow-moving users are allocated to micro cells to save capacity for the macro cells. Using the DS-CDMA concept, micro cells and macro cells share the same frequency. To avoid strong interference at a micro cell from mobile unit in communication with a macro base station, the output power of the mobile unit in the micro cell is increased to overpower the interfering signal. The use of hierarchal cell structure to provide high-quality speech, data communication at rates up to 2 megabits per second and video communication with mobile units traveling at rates in excess of 100 miles per hour and accommodating PCS are seen as needed to meet future mobile telecommunication demands.
In the hierarchal cell structure, the low tier, small cells, e.g., on the order of 100 feet in diameter, accommodate low speeds. The low speed is mostly pedestrian traffic and other traffic moving at speeds below 30 miles per hour. The advantages of small cells is low power, simple, inexpensive and light-weight terminals. What is desirable is an infra structure which allows use of such terminals in all applications, whether in the home or office as a cordless phone, on streets, in shopping malls, airports, etc., and in cars on expressways at highway speeds. Additionally, high-spectrum reuse is needed to provide low-cost, high-quality service which requires a large bandwidth for each subscriber.
To provide wire line toll, quality-voice service, a 32-kilobit per second bit rate is required with ADPCM coders. As wireless data services emerge, even more spectrum bandwidth will be required. In the future, it may be possible to utilize the spectrum in the 60 gigahertz range providing very large amounts of bandwidth. However, the radio wave characteristics at that frequency dictate a very short range, line of site propagation, requiring very small cells. However, as noted, small cells and fast-moving mobile units are incompatible due to the time required for handoff.